The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 2000-032234, filed Feb. 9, 2000.
1. Field of the Invention
The invention relates to an electronic flash device capable of performing dimming control.
2. Description of the Related Art
FIG. 8 is a diagram showing a conventional example of an electronic flash device.
Hereinafter, an overview will be given of the operation of the conventional example with reference to FIG. 8. Initially, a photodiode PD produces electron-hole pairs according to the intensity of emission of the electronic flash device. These electron-hole pairs are separated across the depletion region inside the photodiode PD, and then efficiently led out through an imaginary short between the input terminals of an operational amplifier OP1 to make a photocurrent Ir. This photocurrent Ir flows through a diode D before getting absorbed into the output terminal of the operational amplifier OP1. Here, the output terminal of the operational amplifier OP1 carries the bias voltage V1 dropped by a forward voltage of the diode D. This output voltage of the operational amplifier OP1 is applied to the emitter of a transistor Tr2. Meanwhile, a gain control voltage V2 is applied to the base of the transistor Tr2 through a voltage follower circuit consisting of an operational amplifier OP2.
As a result, the photocurrent which has been logarithmically compressed by the forward voltage characteristic of the diode D is in turn logarithmically decompressed by the (Vbe-Ic) characteristic of the transistor Tr2, whereby a photo-detection current Ip corresponding to the light intensity is restored. Here, increasing/decreasing the gain control voltage V2 allows the gain of the photo-detection current Ip over the light intensity to be adjusted to film speed or the like.
The photo-detection current Ip obtained thus is passed through the loads, or a capacitor C and a resistor Rd, so that it is converted into a photo-detection voltage Vp. This photo-detection voltage Vp is compared with a threshold voltage Vth in a comparator CMP. The comparator CMP, when this photo-detection voltage exceeds the threshold Vth, outputs an emission stop signal STOP to an emission stop circuit (not shown) in the electronic flash device. Incidentally, the transistor Tr1 is a switching circuit for resetting the storage charge in the capacitor C, and is kept short until the point of starting light emission.
In such an operation, modifications to the threshold voltage Vth allow control over the quantity of emission (the integrated quantity of light up to an emission stop) of the electronic flash device.
FIGS. 9(A)-(C) are emission waveforms in the electronic flash device described above. Immediately after the output of the emission stop signal STOP, the emission waveforms keep their light emission with attenuation until complete light-out. The quantity of the remaining light (hereinafter, referred to as xe2x80x9cthe quantity of overrun lightxe2x80x9d) contributes a control error to dimming control.
Conventionally, such a control error has been mended by differential correction using the resistor Rd. Across this resistor Rd occurs in real time a voltage drop corresponding to the light intensity. This voltage drop is added to the storage capacitance in the capacitor c (an integrated value of light intensities, corresponding to the quantity of emission), thereby elevating the photo-detection voltage Vp. Thus the higher the instantaneous light intensities are, the greater the photo-detection voltage Vp appears to be, which leads to earlier output of the emission stop signal STOP. In general, higher emission intensities at the point of emission stop would produce greater quantities of overrun light. Therefore, such differential correction could improve the control error in the dimming control up to a certain degree.
By the way, in weak light emissions, the quantity of overrun light forms a great proportion to the target quantity of emission as shown in FIG. 9(B), with a possible control error of the order of 30%.
Nevertheless, in the conventional differential correction, the resistor Rd could produce only an extremely small voltage drop in weak light emissions, thereby promising little correction effects.
In view of the foregoing problem, an object of the present invention is to provide an electronic flash device which can improve the dimming precision even in weak light emissions.
To achieve this object, the present invention is configured as stated below.
An electronic flash device according to the present invention comprises: an emission unit for performing flash emission; an emission monitoring unit for monitoring the quantity of emission by the emission unit; and an emission control unit for stopping the emission by the emission unit based on a comparison between the quantity of emission monitored by the emission monitoring unit and a predetermined target quantity of emission. Here, the emission control unit predicts the quantity of overrun light after stopping of the emission based on the target quantity of emission and corrects emission stop timing in accordance with the quantity of overrun light.
In the configuration described above, the emission stop timing is corrected based on the quantity of overrun light predicted from the target quantity of emission. Therefore, in contrast to the conventional differential correction, it becomes possible to reliably make a correction to cover the quantity of light overrun, independent of the magnitudes of instantaneous light intensities. This allows a sure improvement to the precision of the dimming control even in weak light emissions.
Here, it is particularly preferable for the emission monitoring unit to receive light from the emission unit directly. In this case, the emission monitoring unit is free from receiving external effects, such as to-subject distances and subject reflectance. Thus, the conditions for the light quantity monitoring remain constant almost each time. This allows predictions to be made without consideration of these external effects, thereby ensuring higher accuracy for the predictions on the quantity of overrun light. Moreover, since the conditions for the light quantity monitoring remain constant almost each time, it naturally follows that corrections when the emission stop timing is advanced improves in accuracy also. These synergistic effects bring about further improvements to the precision of the dimming control.
The emission monitoring unit in the present invention preferably includes: a photoelectric transducer for receiving light from the emission unit to generate an output according to the light intensity; a storage unit for storing the output generated by the photoelectric transducer; a discharge control unit for sequentially discharging a predetermined amount of storage out of the storage unit so that the storage in the storage unit is maintained generally constant; and a counter for counting the number of times the discharge control unit discharges the predetermined amount of storage and for outputting the count result as the result of monitoring the quantity of emission.
The predictions on the quantity of overrun light according to the present invention is generally suitably effected through digital processing, including prediction computing and making table reference. To execute these kinds of digital processing as part of the dimming control in the electronic flash device, it is preferable for the dimming control itself to be digitally controlled.
Nevertheless, digitally converting such high-speed, wide-dynamic-range phenomena as flash emission in real time inevitably requires an A/D conversion circuit with appropriate high speed and performance. On this account, simply realizing a digital dimming control would result in a negative effect that the electronic flash device complicates in configuration and increases in cost.
Thus, in the above-described configuration, the process of an analog feedback control, of maintaining the amount of storage in the storage unit generally constant is utilized to easily convert the quantity of emission into the number of discharges (digital amount). As a result, a dimming control of a digital type is realized in a simple configuration without having any additional high-speed, high-performance A/D conversion circuit.
In particular, such a configuration makes it possible to make a precise, sure correction to the emission stop timing through simple digital processing (e.g. digital processing of offsetting the target quantity of emission or the number of discharges to cover the predicted quantity of overrun light).
The emission control unit in the present invention preferably predicts the quantity of overrun light after stopping of the emission based on the rate between the target quantity of emission and the quantity of full emission by the emission unit (the rate of emission), and corrects emission stop timing in accordance with the quantity of overrun light.
In general, the quantity of full emission varies due to such factors as xe2x80x9cchanges in boosting voltagexe2x80x9d and xe2x80x9cdeterioration of the flash tube due to aging.xe2x80x9d Naturally, this variation in the quantity of full emission also changes the quantity of overrun light so the predicting accuracy of the quantity of overrun light mentioned above inevitably deteriorates.
Therefore, in the above-described configuration, the quantity of overrun light is predicted based on the rate between the target quantity of emission and the quantity of full emission (the rate of emission). Such predictions based on the rate of emission normalized with the quantity of full emission ease the effect from the full emission varying in quantity, thereby preventing deterioration in predicting accuracy.
The quantity of full emission is preferably estimated, for example, from the value of the boosting voltage before emission, from the quantity of the previous emission, records of past emissions, or the quantity of light monitored in preparatory emissions, and so on.
The emission control unit in the present invention preferably corrects the emission stop timing upon each emission when repeating the emitting/stopping by the emission unit a plurality of times to perform split emission.
Most of the individual emissions (chopper emissions) in the split emission are weak light emissions. As described above, the present invention offers higher correction effects in weak light emissions as compared with the conventional example. Therefore, performing the light quantity correction of the present invention in each chopper emission allows a significant improvement to the dimming precision in split emission.
Particularly, the improvement to the dimming precision for each chopper emission allows precise control over the mean intensity of light when taking split light in terms of flat light. Accordingly, it becomes possible to control light exposure with precision in the cases where the exposure period is controlled separately (e.g. where a camera shutter is slit-moved for exposure).
The emission control unit in the present invention preferably predicts the total sum of the quantities of overrun light for the entire split emission when repeating the emitting/stopping by the emission unit a plurality of times for split emission, and corrects the number of emissions in accordance with the total sum of the quantities of overrun light.
In the configuration described above, the light quantity correction is effected by correcting the number of stops in the split emission. Therefore, it becomes possible to control light exposure with precision in the cases where the exposure period is not controlled separately (e.g. where the flash is emitted with the shutter fully open).